1. Field of the Invention
The present invention generally relates to handover implementation in a mobile communication system. More particularly, the present invention relates to a method and apparatus for performing an uplink timing synchronization procedure (UL timing sync procedure) in a User Equipment (UE), upon handover to a target cell.
2. Description of the Related Art
Multiple access schemes are used in wireless communication systems. Among the principal multiple access schemes are Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a special case of MultiCarrier Modulation (MCM) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to multiple orthogonal subcarriers (or subcarrier channels), prior to transmission.
As a substitute for a 3G mobile communication standard, Universal Mobile Telecommunication System (UMTS), a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), a major OFDM system, is a future-generation mobile communication system under discussion in the 3GPP standardization body.
FIG. 1 illustrates the configuration of a 3GPP LTE system.
Referring to FIG. 1, a UE 110 is a 3GPP LTE terminal. An Evolved-Radio Access Network (E-RAN) 140 performs the functionalities of a Node B and a Radio Network Controller (RNC) of the conventional 3GPP system. Node B is a radio device that directly participates in communications with a UE and manages cells. The RNC controls a plurality of Node Bs and radio resources.
As with the conventional 3GPP system where Node B and RNC are configured as separate nodes, E-RAN 140 can be configured as separate physical nodes, Evolved-Node B (E-Node B or E-NB) 120 and Evolved-RNC (E-RNC) 130, or can merge E-NB 120 and E-RNC 130 therein. E-NB 120 and E-RNC 130 are physically merged into the E-RAN 140. Yet, the following description also holds true in the former case.
An Evolved-Core Network (E-CN) 150 can be a node combining a Serving General Packet Radio Service (GPRS) Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) of the conventional 3GPP system. E-CN 150, which is located between a Packet Data Network (PDN) 160 and E-RAN 140, allocates an Internet Protocol (IP) address to UE 110 and functions as a gateway that connects UE 110 to PDN 160.
FIG. 2 is a UL timing sync procedure in the 3GPP LTE system.
Referring to FIG. 2, a first UE (UE 1) is relatively near an E-NB and a second UE (UE 2) is relatively remote from the E-NB.
T_pro1 denotes the propagation delay time of radio transmission from the E-NB to UE 1, and T_pro2 denotes the propagation delay time of radio transmission from the E-NB to UE 2. Because UE 1 is nearer the E-NB than UE 2, T_pro1 is shorter than T_pro2. In the illustrated case of FIG. 2, it is assumed that T_pro1 and T_pro2 are 0.333 μs and 3.33 μs, respectively.
When UE 1 and UE 2 are at different distances from the E-NB but within the cell of the E-NB and UE 1 and UE 2 are powered-on or placed in idle mode, there may be a mismatch among the uplink timing of UE 1, the uplink timing of UE 2, and the uplink timings of UEs within the cell that the E-NB detects. UL@UE#1 201 denotes the uplink transmission timing of an OFDM symbol from UE 1, and UL@UE#2 202 denotes the uplink transmission timing of an OFDM symbol from UE 2. A timing gap or delay exists between UL@UE#1 and UL@UE#2.
The E-NB receives the OFDM symbols UL Frame@EN-B, at timings 212 and 213, with the propagation time delays of uplink transmission from UE 1 and UE 2 as shown. Specifically, the uplink symbol transmitted at the uplink timing UL@UE#1 from UE 1 arrives at E-NB at the timing UL Frame@E-NB 212 and an uplink symbol transmitted at the uplink timing UL@UE#2 from UE 2 arrives at E-NB at the timing UL Frame@E-NB 213. Since UE 1 and UE 2 are not yet synchronized, the timing of starting to decode uplink OFDM symbols, UL Frame@E-NB 211 in the E-NB, the timing of receiving the OFDM symbol from UE 1, UL Frame@E-NB 212, and the timing of receiving the OFDM symbol from UE 2, UL Frame@E-NB 213 are not aligned. Orthogonality is not kept between the uplink symbols from UE 1 and UE2, resulting in mutual interference. The interference and timing discrepancies make it difficult for the E-NB to successfully decode the uplink symbols received from UE 1 and UE 2.
UL timing synchronization is a process of synchronizing uplink timings among UE 1, UE 2 and E-NB. Therefore, when the UL timing synchronization is acquired, the decoding timing, the reception timing from UE 1, and the reception timing from UE 2 are all aligned, as denoted by reference numerals 215, 216 and 217. Thus, the E-NB can succeed in decoding the uplink symbols received from UE 1 and UE 2.
A description will be made below of a UL timing sync procedure among a UE in active mode, a source E-NB that controls a source cell before handover, and a target E-NB that controls a target cell in the LTE system.
Active mode is a mode in which a Radio Resource Control (RRC) connection is kept between a UE and an E-NB. The E-NB can transmit and receive service data to and from the UE as well as signaling for the UE in the active mode. The UE transmits and receives data to and from the E-NB using time resources and radio resources (e.g. frequency) allocated by scheduling information from the E-NB. Handover is a process of enabling the UE to continue receiving on-going dedicated data/signaling when it moves from one cell to another in the active mode.
FIG. 3 is a signal flow diagram for a UL timing sync procedure among a UE in active-mode, a source E-NB, and a target E-NB, upon handover.
Referring to FIG. 3, a UE 301 located in a source cell under control of a source E-NB 302 sends a MEASUREMENT REPORT message to the source E-NB 302 in step 311. The source cell is a cell in which UE 301 is located and which serves UE 301 before handover, and source E-NB 302 controls the source cell. A target cell is a cell to which UE 301 is to move by handover, and a target E-NB 303 controls the target cell. The MEASUREMENT REPORT message notifies the network of radio measurements about the serving cell and neighbor cells to support handover.
By the MEASUREMENT REPORT, UE 301 informs source E-NB 302 that a neighboring cell (i.e. a target cell) in a better radio channel environment than the serving cell (i.e. the source cell) has been discovered. Source E-NB 302 then decides to hand UE 301 over to the target cell based on the information included in the MEASUREMENT REPORT message.
In step 313, a handover procedure is performed between source E-NB 302 and target E-NB 303 to support the handover from the source cell to the target cell. The handover procedure may involve reservation of radio resources for UE 301 in the target cell, allocation of a UE Identification (ID) for use in the target cell, forwarding of a UE context and a service context associated with UE 301 from source E-NB 302 to target E-NB 303, and setup of a data transmission path between source E-NB 302 and target E-NB 303.
When the network has completely prepared for the handover, source E-NB 302 sends a HANDOVER COMMAND message to UE 301, commanding UE 301 to move to the target cell in step 315. The HANDOVER COMMAND message may provide UE 301 with information about the configuration of a radio channel to be used in the target cell.
Upon receipt of the HANDOVER COMMAND message, UE 301 selects a preamble code for use in a UL timing sync procedure with the target cell in step 321. The preamble code is a bit sequence of a predetermined pattern. Preamble code patterns are preset between UE 301 and target E-NB 303 for use in the UL timing sync procedure. UE 301 selects one of the preamble code patterns.
UE 301 sends a UL SYNC REQ message including the selected preamble code to target E-NB 303 in step 323. Target E-NB 303 can find out by the preamble code how far the uplink transmission timing of UE 301 is from the uplink reception timing of target E-NB 303. Hence, target E-NB 303 calculates timing adjustment information (referred to as timing adjustment info) to match the uplink transmission timing of UE 301 to the uplink reception timing of target E-NB 303.
In step 325, target E-NB 303 replies to UE 301 with a UL SYNC RES message on the downlink. The UL SYNC RES message includes the timing adjustment info and a preamble code ID. The preamble code ID identifies that the UL SYNC RES message is a response for the UL SYNC REQ message sent from UE 301.
UE 301 adjusts the uplink transmission timing based on the timing adjustment info included in the UL SYNC RES message in step 331 and sends a UL SYNC REQ message including a preamble code to the target E-NB 303 in step 333. This preamble code may have a different pattern from that of the preamble code sent in step 323.
In step 335, the target E-NB 303 replies with a UL SYNC RES message including timing adjustment info and a preamble code ID.
The procedure for adjusting the uplink transmission timing of UE 301 by transmitting and receiving preamble codes between UE 301 and target E-NB 303 can be repeated until UE 301 acquires the UL timing synchronization.
Upon acquisition of the UL timing synchronization, in other words, if the timing adjustment info included in the UL SYNC RES message in step 335 is made inactive (e.g. 0), UE 301 can send uplink signaling/data to target E-NB 303 in step 341. For example, a HANDOVER COMPLETE message can be sent to target E-NB 303, notifying successful completion of the handover to the target cell in step 341.
As described above with reference to FIG. 3, the UL timing sync procedure performed in the target cell upon handover to the target cell creates uplink/downlink signaling overhead and causes a service delay until the UE starts to send uplink signaling/data in the target cell in the conventional mobile communication system.